1. Field of the Invention
This invention relates to a non-contact type detecting apparatus for detecting the surface position of an object to be detected, and more particularly there is provided an apparatus so adapted that even in a case where a transparent thin film is formed on the surface of an object to be detected, not only the surface position is detected accurately but also the thickness of the thin film is measured.
2. Description of the Prior Art
FIG. 1 shows an example of conventional surface position detecting apparatus. This apparatus detects the surface position of an object to be detected in a projection exposure apparatus and the object 1 to be detected is loaded on a stage 12 which is vertically movable below a projection optical system 30. The object of this apparatus is such that in order to align the surface position of the object 1 with the focal position (the mask pattern projection image plane) of the projection optical system 30 through the level control of the stage 12, the position of the object 1 in the direction of an optical axis AX is detected and the detected information is applied to the level control system of the stage 12. While, in this case, the description has been made by using for example a wafer constituting the object to be exposed as an example of the object 1 to be detected, the apparatus can also be used as a detecting apparatus for detecting the surface position of a work constituting an object to be examined in cases where the distance between a work and a tool or the like is adjusted in any other processing apparatus or the like.
In this detecting apparatus, as shown in FIG. 1, only the useful wavelength component is selected from the light beam from a light source 31 by a filter 32 and then it is used to illuminate a slit plate 34 through a lens system 33. Then, an image of the elongated slit of the slit plate 34 is projected and formed on the surface of the object 1 to be detected by a lens system 35. Further, after the reflection from the object 1 to be detected, the projected image is again formed on a slit plate 37 by an imaging lens system 36. The resulting image is transmitted through the slit of the slit plate 37 and then condensed on a photoelectric detector 39 by a condenser lens 38, thereby producing a detection signal 40. Then, a stage lifter 11 is operated in such a manner that the detection signal 40 attains for example the maximum and the stage 12 and the object 1 loaded on it are positioned in the height direction.
However, where a transparent film 1b is applied onto a reflecting surface la of the surface of the object 1 as shown in FIG. 2, an illuminating light I.sub.o for forming a slit image falls obliquely on the surface of the object 1 so that due to the internal reflections within the transparent film 1b, multiple images due to multiple reflected rays R.sub.1, R.sub.2, - - - are formed on the slit 37. In this case, since their imaging positions on the slit are different from one another, there is a difference in the center of gravity of light quantity between the cases where the transparent material is used and where no transparent material is used.
Therefore, there is a disadvantage that when effecting the position detection, it is difficult to make a distinction between the surface 1a of the object 1 and the surface of the thin film 1b and thus it is impossible to detect any specified surface.